It is conventionally known to provide time reference information in time signals that are transmitted by radio transmission from a time signal transmitter. Such a signal may also be called a time marker signal, a time data signal, a time code signal, or a time reference signal, for example, but will simply be called a time signal herein for simplicity. The time signal transmitter obtains the time reference information, for example, from a high precision atomic clock, and broadcasts this highly precise time reference information via the time signal. Thus, any radio-controlled clock receiving the signal can be synchronized or corrected to display the precise time in conformance with the time standard established by the atomic clock that provides the time reference information for the time signal transmitter. The time signal is especially a transmitter signal of short duration, that serves to transmit or broadcast the time reference information provided by the atomic clock or other suitable time reference emitter. In this regard, the time signal is a modulated oscillation generally including plural successive time markers, which each simply represent a pulse when demodulated, whereby these successive time markers represent or reproduce the transmitted time reference with a given uncertainty.
A time signal transmitter as mentioned above is, for example, represented by the official German longwave transmitting station DCF-77, which continuously transmits amplitude-modulated longwave time signals controlled by atomic clocks to provide the official atomic time scale for Central European Time (CET), with a transmitting power of 50 kW at a frequency of 77.5 kHz. In other countries, such as Great Britain, Japan, China, and the United States, for example, similar transmitters transmit time information on carrier waves in a longwave frequency range from 40 kHz to 120 kHz.
FIG. 1 diagrammatically represents the encoding scheme or protocol of a time code telegram A that pertains for the encoded time information provided by the German time signal transmitter DCF-77. The telegram in this case consists of 59 bits in 59 time frames, whereby each single bit or time frame corresponds to one second. Thus, the so-called time code telegram A, which especially provides information regarding the correct time and date in binary encoded form, can be transmitted in the course of one minute. The first 15 bits in bit range B comprise a general encoding, which contain operating information, for example. The next 5 bits in bit range C contain general information. Particularly, the general information bits C include an antenna bit R, an announcement bit A1 announcing or indicating the transition from Central European Time (CET) to Central European Summer Time (CEST) and back again, zone time bits Z1 and Z2, an announcement bit A2 announcing or indicating a so-called leap second, and a start bit S of the encoded time information.
From the 21st bit to the 59th bit, the time and date informations are transmitted in a Binary Coded Decimal (BCD) code, whereby the respective data are pertinent for the next subsequent or following minute. In this regard, the bits in the range D contain information regarding the minute, the bits in the range E contain information regarding the hour, the bits in the range F contain information regarding the calendar day or date, the bits in the range G contain information regarding the day of the week, the bits in the range H contain information regarding the calendar month, and the bits in the range I contain information regarding the calendar year. These informations are present bit-by-bit in encoded form. Furthermore, so-called test or check bits P1, P2, P3 are additionally provided respectively at the ends of the bit ranges D, E and I. The 60th bit or time frame of the time code telegram A is not occupied, i.e. is “blank” and serves to indicate the beginning of the next time frame. Namely, the minute marker M following the blank interval represents the beginning of the next time code telegram A.
The structure and the bit occupancy of the telegram A shown in FIG. 1 for the transmission of time signals is generally known, and is described, for example, in an article by Peter Hetzel entitled “Zeitinformation und Normalfrequenz” (“Time Information and Normal Frequency”), published in Telekom Praxis, Vol. 1, 1993.
The transmission of the time marker or code information is performed by amplitude modulating a carrier frequency with the individual second markers. More particularly, the modulation comprises a dip or lowering or reduction X1, X2 (or alternatively an increase or raising) of the carrier signal X at the beginning of each second, except for the 59th second of each minute, when the signal is omitted or blank as mentioned above. In this regard, in the case of the time signal transmitted by the German transmitter DCF-77, the carrier amplitude of the signal is reduced, to about 25% of the normal amplitude, at the beginning of each second for a duration X1 of 0.1 seconds or for a duration X2 of 0.2 seconds, for example as shown in present FIG. 2.
These amplitude reductions or dips X1, X2 of differing duration respectively define second markers or data bits in decoded form. The differing time durations of the second markers serve for the binary encoding of the time of day and the date, whereby the second markers X1 with a duration of 0.1 seconds correspond to the binary “0” and the second markers X2 with a duration of 0.2 seconds correspond to the binary “1”. Thus the modulation represents a binary pulse duration modulation. As mentioned above, the absence of the 60th second marker announces the next following minute marker.
Thus, in combination with the respective second, it is then possible to evaluate the time information transmitted by the time signal transmitter. FIG. 2 shows a portion of an example of such an amplitude modulated time signal as discussed above. Note that the total duration of each time frame from the beginning of one dip to the beginning of the next dip or second marker X1 or X2 amounts to 1000 msec or 1 second, while the individual dips or amplitude reductions acting as second markers X1 and X2 respectively have individual durations of 100 msec or 200 msec, i.e. 0.1 seconds or 0.2 seconds, as described above for the German transmitter DCF-77.
The general technical background of radio-controlled clocks and receiver circuits for receiving time signals as generally discussed above are disclosed in the German Patent Publications DE 198 08 431 A1, DE 43 19 946 A1, DE 43 04 321 C2, DE 42 37 112 A1, and DE 42 33 126 A1. Furthermore, the methods and techniques for acquiring and processing the time information from transmitted time signals are disclosed in Patent Publications DE 195 14 031 C2, DE 37 33 965 C2, and EP 0,042,913 B1.
Present-day conventionally available time signal receivers are typically designed and constructed to operate with only a single reception frequency, and thus are adapted to receive only a single time signal transmitted at this single frequency from a particular time signal transmitter, and to decode and evaluate only this single time signal. However, new radio-controlled clocks and receiver circuits for radio-controlled clocks are now being developed, that are to be switchable among plural different frequencies. Thereby, such radio-controlled clocks and receiver circuits thereof are to be designed and adapted to receive and process respective time signals from various different time signal transmitters. Accordingly, these radio-controlled clocks must be able to simultaneously receive plural time signals in the frequency range from 40 kHz to 120 kHz. This requirement poses new problems for the reception, amplification, decoding, and evaluation of the respective time signals.
The various different time signal transmitters around the world, e.g. the official time signal transmitters in Germany, the United States, Great Britain, Japan, etc., respectively transmit their associated time signals at various different frequencies in the above mentioned range from 40 kHz to 120 kHz. For example, the German transmitter DCF-77 transmits at a frequency of 77.5 kHz, the British and US transmitters MSF and WWVB respectively transmit at a frequency of 60 kHz, the Japanese transmitter JJY transmits at a frequency of 60 kHz and a secondary or alternative frequency of 40 kHz, etc. Other time transmitters transmit their respective time signals at still other frequencies. In this regard, the various time signals with different carrier frequencies are typically received with different associated reception signal strengths or signal amplitudes. Namely, for example, the received signal amplitude of a low frequency time signal, e.g. around 40 kHz, is typically lower than the received signal amplitude of higher frequency time signals, e.g. in the range from 60 to 77.5 kHz. This is simply a feature or result of the transmission characteristics of the respective signals at these different frequencies.
After being received, the received time signals are amplified by an amplifier provided in the receiver circuit for this purpose. In this regard it is problematic, however, that the amplifier of the receiver circuit conventionally has a constant fixed amplification factor, so that it always amplifies the respective received time signal with the same amplification, regardless whether the received time signal has a relatively lower received signal strength or amplitude or a relatively higher signal strength or amplitude. Thus, the amplified signal output by the amplifier does not always have the optimum signal level for its further processing.
For example, the comparator of the receiver circuit is sometimes not able to correctly and reliably detect the second markers of the signal without problems, especially in a time signal with a lower signal amplitude, and most especially in the case when the time signal is falsified, obscured, or super-imposed with an interference signal. In such a case, the sensitivity and the accuracy of the receiver circuit and the decoding arrangement are thereby reduced. This can lead to problems and errors in the decoding and the subsequent evaluation of the time data encoded in the time signal.
In order to increase the sensitivity especially for low frequency time signals, it would be possible to design the amplifier of the receiver circuit for the “worst case” scenario, i.e. the situation of amplifying the time signal having the lowest frequency and thus the lowest received signal strength among the possible expected time signals. In other words, the amplifier is designed to constantly provide the highest amplification factor, that would pertain for the received time signal having the lowest received signal level or amplitude. Thereby, it is ensured that a low frequency time signal received by the receiver circuit will be sufficiently amplified, so that the following decoding arrangement and evaluating arrangement will have the desired sufficient sensitivity for achieving an accurate decoding and evaluation. The problem arises, however, that other time signals having a higher transmission frequency and thus typically a higher received signal amplitude, will be amplified at the same high amplification factor, leading to over-amplification of such signals. This has various disadvantages, in comparison to a circuit with a lower amplification factor that would be completely adequate for such received signals having a high received signal amplitude.
Thus, conventional receiver circuits simply provide a fixed higher amplification factor (which is higher than would be necessary for at least some received signals), in order to ensure an adequate reception sensitivity and reliability even for time signals with a low transmission frequency and thus low received signal amplitude. This directly leads to higher costs of the circuit for providing a higher power amplifier, and especially also causes a higher power consumption of the amplifier circuit, because the higher amplification factor requires higher amplifier currents and thus directly a higher power consumption. Especially for radio-controlled clock applications with a local limited energy supply, for example from primary batteries or accumulators (i.e. rechargeable secondary batteries), the power consumption and thus the energy consumption is a decisive criteria. Thus, in the above described systems, the amplifier designed for the “worst case scenario”, leads to a relatively short operating life of the batteries, or the need to frequently recharge the accumulators.